Vinyl acetate production using a catalyst comprising palladium, gold, copper and any of certain fourth metals

ABSTRACT

A catalyst for the production of vinyl acetate by reaction of ethylene, oxygen and acetic acid as reactants comprising a porous support on the porous surfaces of which is deposited catalytically effective amounts of metallic palladium and gold, copper as the free metal or cupric acetate, and a fourth metal selected from the group consisting of magnesium calcium, barium, and zirconium, as its oxide or mixture of oxide and free metal.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to new and improved catalysts for the productionof vinyl acetate by reaction of ethylene, oxygen and acetic acid.

2. Background Information Including Description of Related Art

It is known to produce vinyl acetate by reaction of ethylene, oxygen andacetic acid using a catalyst consisting of palladium, gold, and coppersupported on a carrier. While the process utilizing such a catalyst iscapable of producing vinyl acetate at relatively high levels ofproductivity, any expedient which could possibly result in even greaterproductivity would be very desirable.

The following references may be considered material to the inventionclaimed herein.

U.S. Pat. Nos. 3,775,342 issued Nov. 27, 1973, and 3,822,308 issued Jul.2, 1974, both to Kronig et al., each discloses a method of making vinylacetate catalysts comprising treating a support simultaneously orsuccessively with a solution A containing dissolved salts of noblemetals such as palladium and gold and a solution B containing compoundsable to react on the support with the noble metal salts to form waterinsoluble compounds, treating such water-insoluble compounds with areducing agent to convert the water-insoluble noble metal compounds tothe free metals, washing the catalyst to remove water-soluble compounds,and applying an alkali metal compound e.g. an alkali metal carboxylatebefore or after treatment with the reducing agent. Solution A canoptionally also contain salts of other metals such as magnesium,calcium, barium and copper.

U.S. Pat. No. 5,332,710, issued Jul. 26, 1994, to Nicolau et al.,discloses a method of preparing a catalyst useful for the production ofvinyl acetate by reaction of ethylene, oxygen and acetic acid,comprising impregnating a porous support with water soluble salts ofpalladium and gold, fixing the palladium and gold as a insolublecompounds on the support by immersing and tumbling the impregnatedsupport in a reactive solution for at least 1/2 hour to precipitate suchcompounds, and subsequently reducing the compounds to free metallicform.

U.S. Pat. No. 5,347,046, issued Sep. 13, 1994 to White et al., disclosescatalysts for the production of vinyl acetate by reaction of ethylene,oxygen, and acetic acid, comprising a palladium group metal and/or acompound thereof, gold and/or a compound thereof, and copper, nickel,cobalt, iron, manganese, lead or silver, or a compound thereof,preferably deposited on a support material.

U.S. Pat. No. 5,567,839, issued Oct. 22, 1996, to Gulliver et al.,discloses a method of producing vinyl acetate catalysts including thestep of using a barium "salt", such as barium hydroxide, to precipitatewater-insoluble palladium and gold compounds onto a support prior toreduction with a reducing agent. When barium hydroxide is used asprecipitant, residual barium remains in the finished catalyst.

SUMMARY OF THE INVENTION

In accordance with this invention, a catalyst is provided useful for theproduction of vinyl acetate by reaction of ethylene, oxygen and aceticacid, comprising a porous support on the porous surfaces of which isdeposited catalytically effective amounts of metallic palladium andgold, copper as free metal or cupric acetate, and a fourth metalselected from the group consisting of magnesium, calcium, barium, andzirconium, as its oxide or mixture of oxide and free metal, with any ofthe latter metals being hereinafter referred to as a "fourth" metal.

It is believed that vinyl acetate catalysts under the inventioncontaining catalytically effective amounts of palladium, gold, copperand any of the specified fourth metals perform with relatively highactivity and/or low selectivity to CO₂ and/or heavy ends, such that theuse of such catalysts often result in greater vinyl acetate productivitythan when any of various catalysts known in the art is employed.

DETAILED DESCRIPTION OF THE INVENTION

In preparing the catalysts under this invention, the catalyst supportmaterial is composed of particles having any of various regular orirregular shapes, such as spheres, tablets, cylinders, rings, stars, orother shapes, and may have dimensions such as diameter, length, or widthof about 1 to about 10 mm., preferably about 3 to 9 mm. Spheres having adiameter of about 4 to about 8 mm. are preferred. The support materialmay be composed of any suitable porous substance, e.g., silica, alumina,silica-alumina, titania, zirconia, silicates, aluminosilicates,titanates, spinel, silicon carbide, or carbon and the like.

The support material may have a surface area within the range, forexample, of about 10 to about 350, preferably about 100 to about 200 m²/g, an average pore size in the range, for example, of about 50 to about2000 angstroms, and a pore volume in the range, for example, of about0.1 to 2, preferably about 0.4 to about 1.2 ml/g.

In the preparation of the catalysts of this invention, the supportmaterial may be treated to deposit catalytic amounts of palladium, gold,copper and fourth metal on the porous surfaces of the support particles.Any of various methods for accomplishing this purpose may be used, allof which involve simultaneous or separate impregnations of the supportwith one or more aqueous solutions of water-soluble compounds of thecatalytically active metals. Palladium(II)chloride, sodiumpalladium(II)chloride, potassium palladium(II)chloride,palladium(II)nitrate or palladium(II)sulfate are examples of suitablewater-soluble palladium compounds; an alkali metal, e.g., sodium orpotassium salt of auric(III)chloride or tetrachloroauric(III)acid can beused as the water-soluble gold compound; and cupric nitrate trihydrateor hexahydrate, cupric chloride (anhydrous or dihydrate), cupric acetatemonohydrate, cupric sulfate (anhydrous or pentahydrate), cupric bromide,or cupric formate (anhydrous or tetrahydrate), can be used as thewater-soluble copper compound. Depending on which fourth metal isdesired in the catalyst, the following water-soluble salts are examplesof compounds which can be used for the impregnation of such fourthmetal: magnesium sulfate (anhydrous or hydrated), magnesium acetate(anhydrous or hydrated), magnesium chloride (anhydrous or hydrated), ormagnesium nitrate (hydrated); calcium chloride (anhydrous or hydrated),calcium acetate (anhydrous or monohydrate), or calcium nitrate(anhydrous or hydrated); barium acetate (anhydrous or hydrated), orbarium nitrate (anhydrous); or zirconium sulfate tetrahydrate, zirconiumchloride, or zirconium nitrate (anhydrous or pentahydrate). An alkalimetal salt of tetrachloroauric(III)acid, sodium palladium(II)chlorideand cupric nitrate trihydrate or cupric chloride are preferred salts forimpregnation of gold, palladium and copper respectively because of theirgood water solubility.

In preparing the catalyst, the impregnations of the support materialwith solutions of water-soluble salts of the catalytically active metalsmay be effected by any method known to those skilled in the art.Preferably, however, such impregnations are accomplished by the"incipient wetness" method wherein an amount of water-soluble saltsolution used for the impregnation is from about 95 to about 100 percentof the absorptive capacity of the support material. The concentration ofthe solution or solutions is such that the amounts of catalyticallyactive metals in the solution or solutions absorbed on the support isequal to a desired predetermined amount. If more than one suchimpregnation is carried out, then each impregnation may containwater-soluble compound equivalent to all or only a portion of the amountof one or any combination of the four catalytically active metalsdesired in the final catalyst, as long as the amounts of such metals inthe total of the impregnating solutions absorbed are equal to the finaldesired amounts. In particular, it may be desirable to impregnate thesupport with more than one solution of a water-soluble gold compound, asmore fully described hereinafter. The impregnations are such as toprovide, for example, about 1 to about 10 grams of elemental palladium;for example, about 0.5 to about 10 grams of elemental gold; and, forexample about 0.5 to about 3.0 grams of elemental copper per liter offinished catalyst, with the amount of gold being from about 10 to about125 weight percent based on the weight of palladium. Depending on whichfourth metal is desired in the catalyst and assuming such fourth metalis the only one present, the number of grams of elemental fourth metalper liter of catalyst provided by the impregnation may be, for example,within the following ranges.

magnesium: about 0.1 to about 2.0, preferably about 0.3 to about 1.0;

calcium: about 0.2 to about 4.0; preferably about 0.5 to about 1.5;

barium: about 0.2 to about 5.0, preferably about 0.6 to about 3.0;

zirconium: about 0.4 about to 7.0, preferably about 1.0 to about 3.0;

After each impregnation of the support with an aqueous solution ofwater-soluble salt of a catalytically active metal, the metal is"fixed", i.e., precipitated, as a water-insoluble compound such as thehydroxide, by reaction with an appropriate alkaline compound, e.g., analkali metal hydroxide, silicate, borate, carbonate or bicarbonate, inaqueous solution. Sodium and potassium hydroxides are preferred alkalinefixing compounds. The alkaline compound should be in an amount of, forexample, about 1 to about 2, preferably about 1.1 to about 1.8 times theamount necessary to completely precipitate the cations of thecatalytically active metals present in the water-soluble salts. Thefixing of the metal may be done by the incipient wetness method whereinthe impregnated support is dried, e.g., at a temperature of about 150°C. for one hour, contacted with an amount of solution of the alkalinematerial equal to about 95-100% of the pore volume of the support, andallowed to stand for a period of about 1/2 hour to about 16 hours; orthe roto-immersion method wherein the impregnated support without dryingis immersed in a solution of the alkaline material and is rotated and/ortumbled during at least the initial period of precipitation such that athin band of the precipitated water-soluble compound is formed at ornear the surface of the support particles. In carrying out the fixing ofmetals by roto-immersion, the rotation and tumbling may be carried out,for example, at about 1 to about 10 rpm for a period of, for example, atleast about 0.5 hour, preferably about 0.5 to about 4 hours. Thecontemplated roto-immersion method is disclosed in previously cited U.S.Pat. No. 5,332,710, the entire disclosure of which is incorporatedherein by reference.

The fixed, i.e., precipitated palladium, gold, copper and fourth metalcompounds may be reduced, for example, in the vapor phase with ethylene,e.g., about 5% in nitrogen at about 150° C. for about 5 hours afterfirst washing the catalyst containing the fixed metal compounds, untilit is free of anions such as halide, and drying, e.g., at about 150° C.for about 1 hour, or such reduction may be accomplished before washingand drying in the liquid phase at room temperature with an aqueoussolution of hydrazine hydrate wherein the excess of hydrazine over thatrequired to reduce all the metal compounds present on the support is inthe range, for example, of about 8:1 to about 15:1, followed by washingand drying. Other reducing agents and means for reducing the fixed metalcompounds present on the support may be employed as conventional in theart. The reduction of the fixed palladium, gold and copper compoundsmainly results in the formation of the free metal, although a minoramount of metal oxide may also be present, while the reduction of thefixed fourth metal generally results in the formation of an oxide or amixture of oxide and free metal, depending on reduction conditions andwhich fourth metal is present. In preparations using more than oneimpregnation and fixing steps, the reduction may be carried out aftereach fixing step or after the total of the metallic elements have beenfixed on the support.

As an example of foregoing general procedure, a "separate fix" methodmay be used to fix the catalytically active metallic elements on thesupport and reduce the water-insoluble metal compounds to the desirablefree metallic form. In this method, using the specific proceduresdescribed previously, the support is first impregnated with an aqueoussolution of water-soluble compounds of palladium, copper, and fourthmetal by incipient wetness, and the palladium, copper, and fourth metalare then fixed by treatment with an alkaline fixing solution byincipient wetness or roto-immersion, preferably roto-immersion. Thecatalyst is then dried and separately impregnated with a solution of asoluble gold compound having the amount of elemental gold desired in thecatalyst, and the gold is fixed by treatment with an alkaline fixingsolution by incipient wetness or roto-immersion, preferably incipientwetness. If the gold is to be fixed by the incipient wetness method,such fixing may be combined with the impregnation step by using a singleaqueous solution of soluble gold compound and alkaline fixing compoundin an amount in excess of that necessary to convert all the gold in thesolution to a fixed insoluble gold compound, e.g., auric hydroxide. If ahydrocarbon such as ethylene, or hydrogen is to be used in the vaporphase as reducing agent, the catalyst containing the fixed metalcompounds is washed until it is free of dissolved anions, dried, andreduced with ethylene or other hydrocarbon, or hydrogen, as previouslydescribed. If hydrazine is to be used in the liquid phase as reducingagent, the catalyst containing the fixed metal compounds is treated withan aqueous solution of excess hydrazine hydrate before washing anddrying to reduce the metal compounds to the free metals, and thecatalyst is then washed and dried as described.

Another alternate method of preparing the catalyst is a "modifiedroto-immersion" method, in which only part of the gold is impregnatedwith the palladium, copper and fourth metal in a first impregnation, themetals are fixed by reaction with an alkaline fixing compound byroto-immersion, the fixed metal compounds are reduced to the freemetals, e.g., with ethylene or hydrazine hydrate, with washing anddrying done before an ethylene reduction or after a hydrazine reduction.The catalyst is then impregnated with the remainder of the gold which isfixed on the catalyst using any of the procedures described previously.Preferably the impregnation and fixing are accomplished in a single stepby incipient wetness using a single solution of a water-soluble goldcompound and an appropriate alkaline compound. The added, fixed gold isthen reduced, e.g., with ethylene or hydrazine, after or before washingand drying, as described previously.

Not wishing to be bound by theory, it is believed that an advantageousvariant of the catalyst of this invention comprises a porous support onthe porous surfaces of which is deposited metallic copper in a zonesurrounded by deposits of catalytically effective amounts of metallicpalladium, gold, and fourth metal, none of which is substantiallyintermingled with said copper. This catalyst may be prepared usingvarious techniques of impregnation, fixing and reduction as describedhereinbefore. The use of this catalyst in which the zone of copper issurrounded by the palladium, gold and fourth metal and the copper istherefore less exposed to ambient reactor conditions, contributes to areduction in the loss of copper by volatilization.

Another useful variant of the catalyst of this invention comprises aporous support on the porous surfaces of which are depositedcatalytically effective amounts of metallic palladium and gold, thefourth metal as oxide or mixture of oxide and free metal, and copper ascupric acetate. This catalyst variant is made by first preparing acatalyst precursor comprising a porous support on the porous surfaces ofwhich are deposited catalytically effective amounts of metallicpalladium and gold, and a fourth metal as its oxide, or mixture of oxideand free metal using any of the techniques of impregnation, fixing andreduction described previously. The catalyst precursor is thenimpregnated with an aqueous solution of cupric acetate, eithermonohydrate or anhydrous, preferably by incipient wetness. The catalystis then dried such that the finished catalyst contains cupric acetate inan amount equivalent to, for example, about 0.3 to about 5.0 grams,preferably about 0.5 to about 3.0 grams of elemental copper per liter offinished catalyst

After the catalyst containing palladium, gold and copper in freemetallic form and fourth metal as oxide or mixture of oxide and freemetal, deposited on a support material, is prepared by any of theforegoing methods, it is advantageously further impregnated with asolution of an alkali metal acetate, preferably potassium or sodiumacetate, and most preferably potassium acetate. The catalyst is thendried such that the finished catalyst contains, for example, about 10 toabout 70, preferably about 20 to about 60 grams of alkali metal acetateper liter of finished catalyst. When the catalyst variant is beingprepared in which the copper is present as cupric acetate, the optionalimpregnation of the catalyst with alkali metal acetate, when carriedout, may be accomplished before or after the impregnation with cupricacetate. Preferably, however, the alkali metal acetate impregnation iscombined with that of cupric acetate, i.e., the catalyst containingmetallic palladium and gold, and fourth metal as oxide or mixture ofoxide and free metal, is impregnated simultaneously with a singlesolution of both cupric acetate and alkali metal acetate to yield afinished catalyst which after drying contains the desired amounts ofboth acetates.

While the catalysts of this invention have been described containingonly one "fourth" metal, more than one of such metals can actually bepresent. When at least two of such described "fourth" metals are desiredin the catalyst, the initial impregnating solution will containdissolved salts of these metals to provide such metals in the finishedcatalyst within ranges, the upper and lower limits of each of which is afraction of the limits defined previously on the assumption that only asingle "fourth" metal is present, such fraction being the same as thefraction that the individual "fourth" metal is of the total amount offourth metal in the catalyst.

When vinyl acetate is prepared using a catalyst according to the presentinvention, a stream of gas, which contains ethylene, oxygen or air,acetic acid, and desirably an alkali metal acetate, is passed over thecatalyst. The composition of the gas stream can be varied within widelimits, taking into account explosive limits. For example, the molarratio of ethylene to oxygen can be about 80:20 to about 98:2, the molarratio of acetic acid to ethylene can be about 100:1 to about 1:100,preferably about 10:1 to about 1:10 and most preferred about 1:1 toabout 1:8 and the content of gaseous alkali metal acetate can be about1-100 ppm, relative to the acetic acid employed. The alkali metalacetate may be conveniently added to the feed stream as a spray of anaqueous solution of such acetate. The gas stream also can contain otherinert gases, such as nitrogen, carbon dioxide and/or saturatedhydrocarbons. Reaction temperatures which can be used are elevatedtemperatures, preferably those in the range of about 150-220° C. Thepressure employed can be a somewhat reduced pressure, normal pressure orelevated pressure, preferably a pressure of up to about 20 atmospheresgauge.

An advantageous variant of a process for producing vinyl acetate usingthe catalyst of this invention is the inclusion of a non-halogencontaining copper compound in the feed stream of reactants to theprocess. The non-halogen containing copper compound is preferablysomewhat water soluble or acetic acid soluble and may be, for examplecupric acetate (anhydrous or monohydrate) which is preferred, cupricnitrate trihydrate or hexahydrate, cupric sulfate (anhydrous orpentahydrate), or cupric formate (anhydrous or pentahydrate) and thelike. The amount of the copper compound fed to the reaction can be suchas to provide, for example, about 10 ppb (parts per billion) to about 50ppm (parts per million), preferably about 20 ppb to about 10 ppm ofelemental copper relative to acetic acid in the feed stream. By means ofthis feature, the amount of copper in the cupric acetate of the catalystlost by the catalyst volatilization during long term use is reduced,than when no copper compound is included in the feed.

The following non-limiting examples further illustrate the invention.

COMPARATIVE EXAMPLE A AND EXAMPLES 1 TO 3

These examples illustrate the preparation of catalysts under theinvention by the "separate fix" method, and the advantages of suchcatalysts in the production of vinyl acetate in terms of higher activityand/or lower heavy ends selectivity.

In Comparative Example A which served as a control, a support materialconsisting of Sud Chemie KA-160 silica spheres having a nominal diameterof 5 mm., a surface area of about 160 to 175 m² /g, and a pore volume ofabout 0.68 ml/g., was first impregnated by incipient wetness with anaqueous solution of sodium palladium(II)chloride and cupric chloridesufficient to provide about 7 grams of elemental palladium and 1.39grams of elemental copper per liter of catalyst. The palladium andcopper were then fixed to the support as palladium(II)hydroxide andcupric hydroxide by treating the catalyst by roto-immersion with anaqueous sodium hydroxide solution such that the Na/Cl molar ratio wasabout 1.2:1. The catalyst was then dried at 100° C. for 1 hour in afluid bed drier following which it was impregnated by incipient wetnesswith an aqueous solution of sodium tetrachloroaurate in an amountsufficient to provide the catalyst with 4 grams/liter of elemental gold,and sodium hydroxide such that the Na/Cl mole ratio was about 1.8:1, tofix the gold on the support as auric hydroxide. The catalyst was thenwater washed until chloride free (about 5 hours) and dried at 150° C.for one hour in nitrogen flow. The palladium and auric hydroxides werethen reduced to the free metals by contacting the catalyst with ethylene(5% in nitrogen) in the vapor phase at 150° C. for 5 hours. Finally thecatalyst was impregnated by incipient wetness with an aqueous solutionof potassium acetate in an amount sufficient to provide 40 grams ofpotassium acetate per liter of catalyst, and dried in a fluid bed drierat 100-150° C. for one hour.

In Examples 1 to 3, the procedure of Comparative Example A was followedexcept that the solution of sodium palladium(II)chloride and cupricchloride contained in addition varying amounts of a dissolved salt of afourth metal which was subsequently fixed on the support as thehydroxide together with the palladium(II) and cupric hydroxides andreduced with ethylene to the oxide or mixture of oxide and free metaltogether with the free metallic palladium, copper and gold. The fourthmetal salts were, respectively, calcium chloride (Example 1), bariumchloride (Example 2), and zirconium sulfate (Example 3).

The catalysts prepared as described in Comparative Example A andExamples 1-3 were tested for their activity in the production of vinylacetate by reaction of ethylene, oxygen and acetic acid. To accomplishthis, about 60 ml of each type of catalyst prepared in the examples wereplaced in separate stainless steel baskets. The temperature of eachbasket was measured by a thermocouple at both the top and bottom of eachbasket. Each reaction basket was placed in a Berty continuously stirredtank reactor of the recirculating type and was maintained at atemperature which provided about 45% oxygen conversion with an electricheating mantle. A gas mixture of about 130 l/hr (measured at N.T.P.) ofethylene, about 26 l/hr of oxygen, about 128 l/hr of nitrogen, about 130g/hr of acetic acid, and about 2 mg/hr of potassium acetate, was causedto travel under pressure at about 12 atmospheres through each basket.The reaction was terminated after about 18 hours. Analysis of theproducts was accomplished by on-line gas chromatographic analysiscombined with off-line liquid product analysis by condensing the productstream at about 10° C. to obtain optimum analysis of the end products.

Table I shows for each example the identity and amount in grams perliter of catalyst of the elemental fourth metal as the oxide or mixtureof oxide and free metal in the catalyst (4th Met., g/L) in addition tothe 7 g/L of palladium, 4 g/L of gold, and 1.39 g/L of copper, and theresults of the analysis of the reaction product in terms of percentselectivity of CO₂ (CO₂, % Sel.) and heavy ends, (HE, % Sel.) andrelative activity of the reaction expressed as an activity factor (Act.)which is computer calculated in the following way: The computer programuses a series of equations that correlates the activity factor with thecatalyst temperature (during the reaction), oxygen conversion, and aseries of kinetic parameters for the reactions that take place duringvinyl acetate synthesis. More generally, the activity factor isinversely related to the temperature required to achieve constant oxygenconversion.

                  TABLE I                                                         ______________________________________                                                  4th Met, CO.sub.2   HE                                              Example   g/L      % Sel.     % Sel.                                                                              Act.                                      ______________________________________                                        A         --       7.76       1.34  1.92                                      1                Ca, 0.88                                                                              7.67            1.93                                 2                Ba, 3.0                                                                                8.05                                                                                         2.05                                 3                Zr, 2.0                                                                                7.66                                                                                         1.85                                 ______________________________________                                    

COMPARATIVE EXAMPLE B AND EXAMPLES 4 TO 6

In these examples the procedure of Comparative Example A and Examples 1to 3 were followed respectively, except that the nominal diameter of thesilica sphere support material was 7 rather than 5 mm. Table II givesthe results of these experiments which in Examples 4 and 5 are eachaverages of two experiments run with the same catalyst under identicalconditions.

                  TABLE II                                                        ______________________________________                                                  4th Met, CO.sub.2,  HE,                                             Example   g/L      % Sel.     % Sel.                                                                              Act.                                      ______________________________________                                        B         --       8.19       1.45  1.98                                      4              Ca, 0.88                                                                             9.01          1.27                                                                                2.09                                5               Ba, 3.0                                                                             8.86          1.26                                                                                2.05                                6               Zr, 2.0                                                                              9.86         1.26                                                                                2.22                                ______________________________________                                    

The results of the foregoing experiments as shown in Tables I and IIindicate that the addition of calcium, barium or zirconium to anotherwise identical palladium-gold-copper catalyst prepared by theseparate fix method reduces the heavy ends selectivity and/or increasesthe activity factor of the catalyst when used to produce vinyl acetatefrom ethylene and acetic acid under substantially identical conditions.

EXAMPLES 7 TO 12

These examples illustrate the preparation of catalysts according to thepresent invention by the "modified roto-immersion" method and theresults of the use of such catalysts in vinyl acetate production, in thesame terms as those shown for the catalysts of Examples 1-6.

The same support as used in Comparative Example A and Examples 1-3 wasfirst impregnated by the incipient wetness method with a solution ofpalladium, gold, copper and fourth metal salts sufficient to provide 7grams of elemental palladium, 4 grams of elemental gold, 1.9 grams ofelemental copper and varying amounts of the elemental fourth metal. Thepalladium, gold and copper salts used were the same as in the previousexamples, and the fourth metal salts were zirconium sulfate in Examples7 and 8, barium chloride in Example 9, calcium chloride in Example 10and magnesium sulfate in Examples 11 and 12. The metals were then fixedby roto-immersion in an aqueous solution of about 120% of the amount ofsodium hydroxide necessary to precipitate the palladium, gold, copperand fourth metal, and the latter metals were reduced either withethylene in the vapor phase (5% in nitrogen) at about 150° C. for about5 hours, or in the liquid phase using an aqueous solution of hydrazinehydrate at an excess weight ratio of hydrazine to metals of 12:1. Afterthe reduction, the catalyst was washed until chloride free (about 5hours), dried at 100° C. for 1 hour in a fluid drier, and thenimpregnated by incipient wetness with an aqueous solution of gold saltsufficient to provide the catalyst with 3 additional grams per liter ofelemental gold (for a total of 7), and sodium hydroxide such that theNa/Cl mole ratio was about 1.8:1, to fix the additional gold. Theadditional gold was then reduced with the same reducing agent as used inthe first reduction, as described previously, and the catalyst waswashed, dried, and impregnated with potassium acetate as described inComparative Example A. The catalyst was then tested for its function inthe production of vinyl acetate as described in the previous examples.

Table III gives the identity and amount of fourth metal in the catalystin addition to the about 7 g/L each of palladium and gold, and 1.9 g/Lof copper, the results of the reaction in terms of percent selectivityto CO₂ and heavy ends, and the activity factor, all as shown in Tables Iand II, and in addition, whether the reducing agent (Red. Agent) isethylene (C₂ H₂) or hydrazine (N₂ H₄). The reaction results of Examples7 to 11 are for each example, averages of the results of two experimentsrun with the same catalyst under identical conditions.

                  TABLE III                                                       ______________________________________                                               4th Met,   Red.    CO.sub.2,                                                                             HE,                                         Example                                                                              g/L        Agent   % Sel.   % Sel.                                                                             Act.                                  ______________________________________                                         7     Zr, 2.0    C.sub.2 H.sub.4                                                                       7.43    0.92  1.94                                   8            Zr, 2.0                                                                              N.sub.2 H.sub.4                                                                         8.29                                                                                1.12                                                                                2.28                                9           Ba, 3.0                                                                                N.sub.2 H.sub.4                                                                        8.23                                                                                1.1                                                                                  2.27                              10          Ca, 0.88                                                                               N.sub.2 H.sub.4                                                                         8.33                                                                                1.16                                                                                2.27                               11          Mg, 0.53                                                                               N.sub.2 H.sub.4                                                                         8.03                                                                                1.12                                                                                2.26                               12          Mg, 0.53                                                                              C.sub.2 H.sub.4                                                                          7.98                                                                                0.95                                                                                1.59                               ______________________________________                                    

The results of Table III indicate that the fourth metal-containingcatalysts of this invention prepared by the modified roto-immersionmethod functioned in the production of vinyl acetate from ethylene,acetic acid and oxygen with relatively low CO₂ and heavy endsselectivities.

EXAMPLE 13

This example illustrates the preparation by the modified roto-immersionmethod, and results of the use of a catalyst containing magnesium as afourth metal which is similar to that of Example 11 except that itcontains about 4 rather than about 7 grams/liter of gold.

The procedure of Example 11 was followed using hydrazine as reducingagent, except that only enough sodium tetrachloroaurate was present ineach of the two impregnating solutions to provide 2 grams/liter of goldto the catalyst for a total of 4 grams/liter in the final catalyst Whentested in the production of vinyl acetate as described in the previousexamples, the product stream exhibited a carbon dioxide selectivity of9.51%, a heavy ends selectivity of 0.72% which is particularly low, andan activity factor of 1.87.

EXAMPLE 14

This example illustrates the preparation and results of the use of acatalyst in which palladium, gold, and magnesium are deposited on ametallic copper containing support material by the modifiedroto-immersion method such that there is substantially no interminglingof copper with the other metals. It is believed that the copper on thesupport material is surrounded by and not intermingled with the othermetals. This in turn minimizes the loss of copper by volatilization.

A support material as described in comparative Example A in which thespheres had a nominal diameter of 7 mm, was impregnated by the incipientwetness method with an aqueous solution of cupric nitrate trihydratesufficient to provide the catalyst with about 1.9 grams/liter ofelemental copper. Without drying, the copper was fixed on the support bytreating the support by roto-immersion with an aqueous solution ofsodium hydroxide containing about 120% of the amount of sodium hydroxideneeded to convert the copper to cupric hydroxide. The fixed cuprichydroxide-containing support was then water washed until free of anions,dried at a temperature of 100° C. for 1 hour in a fluid bed drier,calcined by heating in air at about 200° C. for about 18 hours, and thecupric hydroxide reduced to metallic copper in the vapor phase bycontact with ethylene (5% in nitrogen) at about 150° C. for about 5hours. The copper-containing support material was then treated todeposit about 7 grams/liter each of palladium and gold, and about 0.53gram/liter of magnesium by the modified roto-immersion method using thetechniques of impregnation, fixing and reduction as described inExamples 7-12 and impregnated with potassium acetate as described inComparative Example A. When tested for its function in the production ofvinyl acetate as described in Comparative Example A, the productexhibited carbon dioxide and heavy ends selectivities of 8.38% and 1.07%respectively. The activity factor was 2.1.

We claim:
 1. A process for the production of vinyl acetate by reactionof ethylene, oxygen and acetic acid as reactants comprising contactingsaid reactants with a catalyst comprising a porous support on the poroussurfaces of which are deposited catalytically effective amounts ofmetallic palladium and gold, copper as the free metal or cupric acetate,and a fourth metal selected from the group consisting of magnesium,calcium, barium, and zirconium, as its oxide or mixture of oxide andfree metal.
 2. The process of claim 1 wherein said fourth metal ismagnesium.
 3. The process of claim 1 wherein said fourth metal iscalcium.
 4. The process of claim 1 wherein said fourth metal is barium.5. The process of claim 1 wherein said fourth metal is zirconium.
 6. Theprocess of claim 1 wherein said catalyst contains an alkali metalacetate deposited on the catalyst.
 7. The process of claim 6 whereinsaid alkali metal acetate is potassium acetate.
 8. The process of claim7 wherein potassium acetate is fed to the reaction together with saidreactants.